Method of storing volatile liquids



W WWW? H. C. BOARDMAN METHOD OF STORING VOLATILE LIQUIDS Filed Sept. 17,1945 March 13, 1951 Patented Mar. 13, 1951 TENT OFFICE METHOD OF STORINGVOLATILE LIQUIDS Harry 0. Boardman, Chicago, Ill., assignor to ChicagoBridge & Iron ompany, a corporation of Illinois Application September17, 1945-,Serial. No. 616,799

6 Claims. (01.220-85) .This invention relates to a method ofoperating astorage vessel, and particularly to a method of operating a storagevessel for liquid such as gasoline which has a substantial vaporpressure which is ,normally substantially below atmospheric pressure.

Theproblem of storing such a volatile liquid has been dealt with in thepast in many ways. When stored in ordinary vessels, th r is no pro:- lemso long as the vessel is completely full. Upon emptying and refilling,however, there is evaporation into the vapor space. Normally the vaporspace is left open to atmosphere so that air enters and is forced outcarrying with it a substantial quantity of gasoline.

In order to overcome this difiiculty, a large number of mechanicalexpedients have been suggested, including floating roofs, breather.roofs, lifter roofs, and the like. All of these present mechanicaldifliculties and are expensive to operate.

The present invention permits the operation of a storage vessel withfixed walls. In accordance with it. a storage vessel is madesufficiently strong to withstand substantially full atmospheric pressuredirected inwardly and to withstand an internal gauge pressuresubstantially equal to atmospheric pressure. The vessel, when filled thefirst time, is permitted to vent to atmosphere to remove substantiallyall air, and the tank is preferably filled to the top. After filling,about 2% of liquid is withdrawn to allow for expansion but no air isvented into the space thus vacated. Furthermore on removal of liquidfrom the vessel, no air is acmitted thereto, liquid being re moved bymeans of a pumping system capable of operating against full vacuum (orfull vacuum less the vapor pressure of the liquid). On filling thevessel no venting is contemplated. However, venting may be requiredwhenever the safety limit of the tank is exceeded. Normally such ventingwill not be required, but excessive pressure may be developed in threeways: (1) By excessively high temperatures: (2) too rapid filling; and(3) by reason of the additive vapor pressure of air entrapped orentrained in the liquid.

vapors therein will be compressed. Theoretically,

these vapors will condense when they reach apressure exceeding maximumvapor pressure, but this does not always follow immediately, andparticularly is not necessarily true where the vessel is filled withextreme rapidity and in the presence of air; Normally I prefer to fillthe vessel in a manner so that pressure generated in the gasolinevapors-will remain low, the excess pressure generated by compressionbeing taken up by condensation in the liquid.

I centration of air occurs for two reasons.

When the vessel is filled it is obvious that.

It is of course impossible to remove all air entrapped 0rentrained bythe liquid and, therefore, air will accumulate slowly within the vessel.

This air not only adc's its vapor pressure to that of the gasolinevapors, but it interferes with the prop r cond nsation of the gasolineduring filling and ultimately requires removal. For this reason Iprovide a vent which operates at nearly the safety limit of the tank forremoving excessive air. The air obviously carries with it vapor, theloss of which is undesirable. This loss is kept to a minimum by ventingwhen the safety limit of the tankhas been approximately reached, and mayfurther be reduced by cooling or refrigerating the vented air in anysuitable manner and returning condensate to the tank.

The use of a vent valve operating at the safety limit of the tank is notonly important because venting is limited to the minimum, but because itinsures that the concentration of air in the gas vapor will normallyhave been markedly increased prior to venting. This increase in con- Inthe first place the vapor pressure of the gasoline is fixed at anytemperature. Therefore, if the tank is set to vent at 12 lbs. and thevapor pressure of the gasoline is, say, 5 lbs. absolute, air will haveto represent approximately 22 lbs. of the total pressure before anyventing occurs. TherefOre increase in pressure represents air, and thehigher the pressure of venting, the greater is the proportion of air,other things being equal.

Furthermore, venting will nearly always occur during-filling of the tankand seldom while the tank is quiescent. During filling of the tank, thepressure will rise, as any residual air is compressed, but at the sametime gasoline vapors within the vapor space will condense because theircondensation pressure is exceeded the minute the tank begins to fill.Therefore, the vapors gradually become more concentrated in air as thefilling operation goes on, and by making the venting pressure as highaspossible, the concentration'of-air increases and less gasoline vaporsare lost upon venting.

It is preferred, as stated before, to fill the tank completelyfullWhenever it is filled, and then to withdraw some liquid and leave about2% of vapor space. This, however, is not customary refinery practice,the ordinary practice being to fill about 98% full. In order to conformto this practice, it is, therefore, desirable in some instances to 3include a second valve which is sealed against operation until the tankis substantially 98% full, and then opens slightly below the safetylimit of the tank and preferably at a pressure very slightly below thefirst valve, and which then stays open to vent all of the gasescontained within the 2% space until atmospheric pressure has beenreached. Such a valve is of value because it vents gases at a time whenthe concentration of air therein is at a maximum. This maximum .c

content of air is due to the fact that gasoline vapors have beencondensing during all of the filling of the tank, the air concentrationincreasing during all of that time. By venting as much as possible whenthe air concentration is at its maximum, vapor losses are reduced.

A typical storage system for storing volatile liquids according to themethod of this invention is shown in the accompanying drawings. Of thedrawings Fig. 1 is an elevation of the storage system; and Fig. 2 is anenlarged fragmentary vertical section through the valves on the top ofthe storage vessel. g

The storage system shown in the accompanying drawings comprises astorage vessel ill, a pump l l,

a' valve i2 set at a desired pressure such as 12 lb. per sq. in., and asecond valve i3 preferably set at a slightly lower pressure than thefirst valve l2. The second valve 13 is designed to open at, for example,11% lb. per sq. in., and to close at substantially atmospheric pressure.

The storage vessel comprises vertically aligned intersecting spheres l4supported by a steel rim I5 which rests upon a concrete foundation [6.This intersecting spherical vessel is of the type shown and described inmy copending application Serial No. 608,884, filed August 4 1945.

The first valve i2 at the top of the storage vessel comprises a valveseat l6 held in closed position by a spring 11 resting against crossbars l8. This valve may be set at any desired pressure, such as 12 lb.per sq. in.

The second valve [3 at the top of the storage vessel includes a firstvalve seat l8 connected by means of a rod 29 to a second valve seat 2|of larger area than the first valve seat [9. Both valve seats are heldin place by means of a spring 22 bracing against cross bars 23. 'Theopening 2d leading to the second valve [3 is normally kept closed bymeans of a gate 25. This gate is opened only when the vessel is slightlyless than full, for example, 98% full, and is opened by means of a float28, which turns the gate 25 about hinge 21, when the vessel is almostfilled.

The outlet from the valves may be joined to a pipe 28, where the vaporsmay be conducted to a refrigerating unit to recover the volatile liquidcarried off by the vapors.

An important part of the storage system is a means for filling andemptying the vessel. This filling and emptying means may be a pump IIwhich must be of suiiicient power to fill the vessel against internalpressure and to empty the tank against full vacuum or full vacuum lessthe vapor pressure of the liquid). Liquid-is led to the pump by means ofan intake line 29 and exhausted to the vessel through a line 38. Whenthe pump is used to withdraw liquid from the vessel these lines serve asan outlet.

Having described my invention in considerable detail, it is my intentionthat the invention be not limited by the details of description unlessotherwise specified, but rather be construed broadly within its spiritand scope as set out in the accompanying claims.

I claim:

1. The method of filling, and emptying a storage tank ior volatileliquids in which said liquids have a substantial vapor pressure normallysubstantially below atmospheric pressure which comprises introducingsuch liquid to a storage vessel capable of withstanding substantially afull vacuum and also an internal gauge pressure substantially equal toatmospheric pressure, continuing the filling of said vesselsubstantially to completion to compress the entrapped air and condensethe vapors, and venting the entrapped air during the filling at apressure not substantially below the safety limit of the vessel, saidvessel thereafter being kept closed to the atmosphere at all timesincluding subsequent emptying and refilling operations except whennecessary to vent air that has been entrained in the entering liquid.

' 2. The method of claim 1 wherein the liquid is introduced into thevessel at such a rate that the pressure of the volatilized gases withinthe vessel is substantially the same as the vapor pressure of the liquidduring the filling operation.

3. The method of claim 1 wherein the liquid is withdrawn from the vesselat such a rate that the pressure of the volatilized gases within thevessel is substantially the same as the vapor pressure of the liquidduring the withdrawing operation.

4. The method of claim 1 wherein the liquid is introduced into andwithdrawn from the vessel at such a rate that the pressure of thevolatilized gases within the vessel is substantially the same as thevapor pressure of the liquid during the filling and emptying operations.

5. The method of claim 1 wherein the venting begins when the vessel isslightly less than full.

. 6. The method of filling, and emptying a storage tank for volatileliquids in which said liquids have a substantial vapor pressuresubstantially below atmospheric pressure which comprises introducingsuch liquid to a storage vessel capable of withstanding substantially afull vacuum and Y also an internal gauge pressure substantially equalphere at all times including subsequent emptying and refillingoperations except when necessary to vent air that has been entrained inthe entering liquid.

HARRY C. BOARDMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,666,935 Howell Apr. 24, 19281,890,870 Tokheim 1 Dec. 13,1932 2,037,731 Mobley Apr. 21, 19362,313,997 Jackson Mar; 16, 1943 2,333,792 Jackson Nov. 9, 1943 2,341,044Jackson Feb. 8, 1944 2,376,263 Marner May 15, 1945

